Time travel is a staple of science fiction. The most accepted model of physics merges time with distance in a spacetime continuum, and gives no preference to the direction of time. Yet the observable universe, everything we see and are familiar with, definitely prefers one direction of time, toward what we call the future. The direction from past to future was referred to as the arrow of time by Ludwig von Boltzmann, who defined entropy as the disorder of an isolated system. He stated  that the entropy of an isolated system always increases or remains constant. Simply put, the arrow of time points from a glass vase falling off of a tabletop toward the shattered vase on the floor in a puddle of water. It’s easy to see a whole vase can become shattered shards, not so easy to envision how the  shattered shards on the floor spontaneously reassemble into the whole vase full of water on top of the table and a dry floor given only the passage of time. In the macroscopic world time definitely only flows one direction.

The problem is that the mathematical description of the paths of the individual particles that make up the vase from table to floor to shards in a puddle works equally well as a description of the paths of the individual particles from shards in a puddle to vase hitting the floor to vase on the table. You only need to reverse the time coordinate. We’ve all seen movies run backwards, which shows how that would work. But if you watch a movie of the vase falling it’s easy to tell which is the actual real-world choice of time coordinates. If you walked into a room and saw a vase spontaneously reassemble and jump from floor to table you’d be astonished. If you saw the vase fall and break you would perhaps be disappointed but not shocked.

In the standard model of quantum physics in relativistic spacetime, space and time are both reversible. Despite its nonintuitive nature, quantum physics makes many predictions that are easy to test and that support its validity. It is eerily correct in a multitude of ways. So there’s a conundrum. Most physicists postulate that the reversibility of time is fundamental and the  growth of entropy is not. So we seem to have an illusion of the passage of time in only one direction, toward greater entropy, greater disorder. The important aspect of illusions is that they point out problems with the underlying model.

The standard model also includes the “Big Bang”, an explosion about 14 billion years ago when the protouniverse was in an extremely low entropy state, with everything in the universe tightly packed into an extremely small space. Sean Carroll is one of the most well-known physicists who have wrestled with the nature of time and  its arrow. He says the question is not why does entropy increase, but rather why was the entropy ever extremely low to begin with?

The way Carroll and most physicists deal with the peculiarities of our universe is to state that the universe is just one of an infinite number of possible universes, each with its own arrow of time and other properties. Since these universes are by definition outside of our own, it’s not clear how we could ever test this hypothesis. But physicists are unwilling to make predictions based on an otherwise spectacularly successful theory unless the predictions  can be falsified.

Carroll puts this into the context of the missing quantum theory of gravity. The standard model of physics provides a unified theory for three of the four forces that govern the rules of particle motion. The strong nuclear force, weak nuclear force and electromagnetic force all have very successful quantum descriptions; gravity does not. If we can develop a good theory of quantum gravity so all the forces are part of a single, self-consistent description, Carroll believes we could more confidently predict things we cannot observe.

What is often neglected is that the universe has enormous numbers of islands of low entropy in a sea of steadily increasing entropy. We call them stars.  Local inhomogeneities in the density of a uniform cloud of interstellar gas lead to gravitational clumping. The entropy of a gas cloud increases as it collapses into a star, the star itself is a location of low entropy, i.e., a high degree of order. On a larger scale, galaxies like the Milky Way and the Andromeda galaxy represent clusters of low entropy objects within a universe of increasing entropy. All of these low entropy objects exist because of gravity. Within the nearest such locale, the solar system, entropy is increasing in the system as a whole but the system was brought into a much lower entropy initial state by gravity.

So what does this mean for time travel? Conceptually time travel is consistent with the Standard Model, although causality takes a hit if the future can cause actions in the past. Without causality the assemblage of shards into a vase would be interesting but not impossible. With causality, all the familiar paradoxes occur. Only if the time traveler’s actions are taken into account (i.e., already known in some sense) do both time travel and causality coexist. So, if a mysterious stranger got your grandparents together, you could be that mysterious stranger, but from your perspective, once you arrived in the past your actions would be constrained in some way.